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Tropical Depression Thirteen Forecast Discussion Number 4
2020-08-20 22:53:17| National Hurricane Center (Atlantic)
Issued at 500 PM AST Thu Aug 20 2020 000 WTNT43 KNHC 202053 TCDAT3 Tropical Depression Thirteen Discussion Number 4 NWS National Hurricane Center Miami FL AL132020 500 PM AST Thu Aug 20 2020 Recent late afternoon visible satellite imagery has shown evidence of a low-level swirl that is racing away from the persistent area of deep convection. It is unclear if that is the only center of circulation or a swirl rotating around the broader circulation. The initial position for this advisory is a compromise between the earlier estimated center location and the swirl seen in satellite imagery. The initial intensity has been held at 30 kt, based on the earlier ASCAT data. The initial motion estimate is a somewhat uncertain 290/18 kt. The cyclone is located south of a subtropical ridge that is forecast to build westward over the western Atlantic through early next week. This ridge should steer the system west-northwestward over the next several days, and the suite of dynamical track models continue to be agreement on this overall scenario. The lastest NHC track foreast is similar to the previous advisory and again lies between the HFIP corrected consensus and the TVCA multi-model consensus. It should be noted that since the system is still lacking in organization, there could be some center reformations that result in some shifts in the track forecast. Although the depression has changed little in strength since it formed yesterday, the overall environment ahead of the system favors gradual strengthening. The cyclone is forecast to remain over warm water and in an area of light to moderate vertical wind shear. Most of the guidance suggests a little more favorable upper-level wind pattern once the system is north of the Greater Antilles, but a track farther south would result in more land interaction, which increases the intensity forecast uncertainty. The updated NHC intensity forecast shows a slightly slower rate of strengthening over the next couple of days, but is unchanged at the latter periods. The overall confidence in both the track and intensity forecasts remain lower than normal. Key Messages: 1. Tropical storm conditions are possible across portions of the northern Leeward Islands, the Virgin Islands, and Puerto Rico Friday night and Saturday, and Tropical Storm Watches have been issued for some of these islands. Heavy rainfall is likely across this area beginning late Friday and could cause mudslides and flash and urban flooding through Sunday. 2. The details of the long-range track and intensity forecasts are more uncertain than usual since the system could move over portions of the Greater Antilles this weekend. However, this system could bring some storm surge, rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida this weekend and early next week. Interests there should monitor this system's progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 20/2100Z 16.7N 53.9W 30 KT 35 MPH 12H 21/0600Z 17.5N 57.2W 30 KT 35 MPH 24H 21/1800Z 18.3N 60.5W 35 KT 40 MPH 36H 22/0600Z 19.1N 63.5W 40 KT 45 MPH 48H 22/1800Z 20.0N 66.7W 45 KT 50 MPH 60H 23/0600Z 20.9N 70.4W 50 KT 60 MPH 72H 23/1800Z 22.2N 74.1W 60 KT 70 MPH 96H 24/1800Z 25.1N 80.4W 65 KT 75 MPH 120H 25/1800Z 28.3N 84.2W 65 KT 75 MPH $$ Forecaster Brown
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